Step 15: Darken the right side of the shape on the right for the sperm whale's tail. Make the lines curvier. Curve the bottom line up toward the tip. At the top add an extra curved line to make the shape thicker.
Step 16: Darken the left side of the initial guide to finish the shape of the sperm whale's tail. Make the lines curvier.
Step 17: At the top of the second initial circle, draw a curved line for the sperm whale's dorsal hump. Draw more curved lines between the first hump and tail for the ridges on the back. The ridges should decrease in size the closer to the tail they are. Draw a couple of curved lines along the bottom for the ridges on the whale's underside.
Step 18: For a cleaner look, erase as much as you can of the initial guide lines. Don't worry about erasing all of the guides. It's okay to leave some behind. Re-draw any final sketch lines you may have accidentally erased.
Final Step: Add some shading to your drawing for extra detail. Sperm whales aren't too colorful, so just apply a medium value as a base for the whole body. Use a slightly darker value to shade the hump and ridges on the back. Use a very light value around the open mouth and around the eye. Add a slightly darker value throughout the body to give the figure more structure and shape. For a more detailed guide on how to shade, check out this tutorial: How to shade.
Add the value lightly at first and gradually build up to the level of darkness that you like. Use a dark value at the bottom for a shadow that gives the sperm whale more dimension and volume. It's a good idea to use reference as you draw for a more accurate depiction of a sperm whale. Don't forget to pause the video after each step to draw at your own pace.